This invention relates generally to drive circuits and specifically to low power drivers for liquid crystal display technologies.
The demand for Liquid Crystal Displays (LCD) continues to exceed supply. LCD""s are implemented as screens on almost all types of digital devices, including watches, personal computers, video monitors, portable computers (e.g., laptops, notebooks, handheld, palm) and projection displays. The size of the display area has steadily grown while general performance of LCD""s has steadily improved in the last years. But an important issue is the power dissipation of the growing LCD""s.
Users are steadily looking for increased display size and higher resolution. Enhancing both of these features, however, consumes more and more energy. New designs for portable digital devices, in particular, are aiming at lowering the power dissipation of every component and therefore increasing battery life.
Among the different factors contributing to the power dissipation of an LCD are the background illumination and the signal or image information transfer. The background illumination can be completely eliminated in applications where the natural incident light can be used so that the LCD operates in a reflection mode. In one aspect, this invention relates to the power reduction related to the signal or image information. This signal information transfer is related to the charging and discharging of a matrix of capacitive LC-pixels.
The most popular and most widely used LCD""s are based on Twisted Nematic, Super Twisted Nematics and Cholesterics. Displays fabricated with these kinds of LCD-materials operate with polarizers and analyzers, hence restricting the use of back light free operation. This induces optical losses such that more power is needed for the back light illumination or higher levels of natural incident light are required.
More recently much effort has been spent in the development of Polymer Dispersed LCD""s as described by Ikeno et al., xe2x80x9cA 23-cm Diagonal Bright Reflective Guest-Host TFT-LCDxe2x80x9d, SID 1995 Digest, pp. 333-336. These Polymer Dispersed LCD""s do not use polarizers, thereby saving back light power or allowing a lower level of natural light illumination. Unfortunately, the driving voltages for the Polymer Dispersed LCD pixels are higher and therefore any energy saved from lower power back light illumination is lost. The present invention can drastically lower the power dissipated when driving the pixels even at extended voltage levels, such that eventually the LCD consumes less energy.
Several methods, or addressing schemes, have been developed for sending signals or image information to LCD""s. The three most important are : direct addressing, and passive and active matrix addressing. Direct addressing, usually used in watches and calculators, is great for simple alphanumeric characters, since one signal controls one segment of pixels. However, direct addressing is unrealistic for larger systems because of the large number of wires that need to be interfaced.
In a matrix system, the number of wires can be greatly reduced by splitting up the display into a grid of wires called rows and columns, with a pixel at the intersection of each row and column. Matrix displays can be grouped into two categories, passive matrix liquid crystal displays (PMLCD) and active matrix liquid crystal displays (AMLCD).
A PMLCD is the simplest display for achieving low power, low cost and small size. In a PMLCD, only a LC-pixel is located at the intersection of each column and row. PMLCD""s have, in general, less performance than the AMLCD""s but are much simpler to fabricate and therefore preferred for smaller, less accurate displays. In an AMLCD, an extra nonlinear element is introduced at each pixel location to enhance the nonlinear behavior (i.e., contrast) of each pixel. This extra nonlinear element can be a two-terminal device or a three-terminal device. The number of terminals at the pixel location influences the driving scheme.
The trend toward larger, higher definition displays in notebook computers is forcing display manufacturers to seek new electrical drive methods for the integrated circuit that drives the LCD. Current methods for driving the electrical signals onto these displays have been proposed to address significant issues with power dissipation and image quality.
For example, Erhart et al. (xe2x80x9cCharge-Conservation Implementation in an Ultra-Low-Power AMLCD Column Driver Utilizing Pixel Inversionxe2x80x9d, SID 1997 Digest, pp. 23-26) implemented a capacitively based energy recovery method for AMLCD displays. At the beginning of each row time, the column busses are shorted together to a supplemental capacitor, which naturally maintains a potential halfway between average upper and average lower voltage. The maximum power saving of this method is limited to 50%.
Okumura et al. (xe2x80x9cMultifield driving method for reducing LCD Power dissipationxe2x80x9d, SID 1995 Digest, pp. 249-252) proposed a multi-field driving method for reducing LCD power dissipation. In this method, the image refresh rate is lowered without flicker occurrence by dividing the field image into an odd number of interlaced sub-field images. One sub-field flicker is compensated by the other sub-field flickered images. The power reduction is here limited to 30%.
In another proposal formulated by Sakamoto et al. (xe2x80x9cHalf-Column-Line driver method for Low-Power and Low-Cost TFT-LCDsxe2x80x9d, SID 1997 Digest, pp. 387-390), the number of column drivers is halved and the number of row drivers doubled. This technique can lead again to a power reduction of 50%.
The driving power of the LCD""s schemes for two terminal devices has been improved by increasing the number of voltage levels applied to the select line as outlined by R. A. Hartman (xe2x80x9cTwo-Terminal Devices Technologies for AMLCDsxe2x80x9d, SID 1995 Digest, pp. 7-9). The excellent image quality demands higher power dissipation. The system of the present invention is compatible with these improved schemes but further reduces the power dissipation.
In some cases, panel manufacturers are returning to direct drive displays. Direct drive refers to the ability of the column driver chips to xe2x80x9cdirectlyxe2x80x9d provide the alternating voltage and the variable magnitude. See, for example, Erhart et al. (xe2x80x9cCharge-Conservation Implementation in an Ultra-Low-Power AMLCD Column Driver Utilizing Pixel Inversionxe2x80x9d, SID 1997 Digest, pp. 23-26). This early drive technique had been abandoned by many of the major LCD manufacturers due to cost concerns and replaced by common backplane node driving. :Although direct drive requires higher voltage driver circuits, substantial power dissipation and image quality improvement could be reached compared to traditional drive methods. The complementary driving schemes, direct drive and common backplane node, can both benefit from the driving circuit and method described herein. But even the prior art methods proposed to date have not provided satisfactory reduction of power dissipation.
The cost of the LCD is partially influenced by the glass quality and the integration possibility of the peripheral driver circuits on the LCD substrate. This is discussed by Stewart et al., xe2x80x9cCircuit Design for a-silicon AMLCDs with Integrated Driversxe2x80x9d, SID 1995 Digest, pp. 89-92 and Aoyama et al., xe2x80x9cInverse Staggered poly-Si and Amorphous Si Double Structure Thin Film Transistors and LCD Panels with Peripheral Driver Circuits Integrationxe2x80x9d, IEEE Trans. Elect. Devices 43(5), pp. 701-705 (1996). Drivers and nonlinear elements integrated on poly-Si substrates feature low resistances but also require expensive high-quality glass resistant to high temperature processing. The technological tendency has been toward laser annealed hydrogenated amorphous silicon (a-Si:H), which features low resistance values and process temperatures and therefore cheaper glass. The invention proposed here can strongly benefit from these technological improvements as explained below.
In one aspect, the present invention proposes a driving system where the pixels of a LCD or similar device are charged and discharged by constructing a LRC resonant circuit whose oscillation can be interrupted after half an oscillation period (or after an even number of full periods). The energy used for charging a pixel is partially recuperated when discharging the pixels. The energy recuperation improves with the decrease of the resistance of the drivers and the nonlinear elements in the AMLCD""s. The proposed driving circuit and methods of this embodiment will continue to benefit from these technological tendencies.
In another aspect, the present invention is directed toward a novel apparatus and method for charging and discharging the pixels of a matrix-based liquid crystal display. The power dissipation is reduced without sacrificing the quality of operation of the liquid crystal display matrix.
The present invention also provides an oscillation sensing means and a method to sense the state of the oscillation such that the oscillation can be interrupted at the appropriate time.
In one aspect, a row driver circuit can be used with a matrix display device that includes a plurality of pixels disposed in rows and columns. The row driver circuit includes at each row a first and second switch with their current path coupled to a positive and negative high voltage node, respectively. A third switch at each row is coupled with its current path to the ground. A fourth switch at each row enables or disables the oscillation of the resonant row circuit, comprising a common inductive element connected to common switches. A first common switch couples the common inductive element to half the positive high voltage node. A second switch couples the inductive element to half the negative high voltage node. Variants on this scheme will be detailed later.
In another aspect, a column driver circuit can be used with a matrix display device that includes a plurality of pixels disposed in rows and columns. The column driver circuit includes at each column a first and second switch with their current path coupled to a positive and negative high voltage node, respectively. A third switch at each column connects or disconnects the said. column to a common resonant circuit, consisting of a common inductive element connected at one side to ground and at the other side to the common node of the said third switches. A matrix display can use either one or both of these column driver circuits and row driver circuits.
Depending on the matrix technology used, the novel driving methodology can be adapted. In the preferred embodiment, different driving schemes are proposed for passive matrix (PMLCD), two-terminal active matrix, and three terminal active matrix (AMLCD). Examples of these embodiments are described in the next paragraphs.
Columns/Passive Matrix and Three Terminal Active Matrix. In this embodiment, the driving scheme allows a subset of columns of pixels to be connected together thereby reversing their polarity from plus to minus in a first step and from minus to plus in a second step. In this embodiment, the polarity change for each group of pixels is established in a sequential way, by connecting them to an inductive element whose voltage node is biased at a voltage level between the opposite polarity voltage levels. Energy stored in a capacitive form on one such group of connected columns is transferred to the inductive energy storage element and then back towards the capacitive pixels. Snap circuits can be employed to snap the voltage to the required voltage level after the non-perfect voltage change occurs.
Rows/Passive Matrix, Two-Terminal and Three-Terminal Active Matrix. In this embodiment, the driving scheme allows the pixels (or the gates) of each row to charge in turn from the deselecting voltage level toward the selecting voltage level via an inductive storage element whose voltage node is biased at a mid-level voltage. When the capacitive energy is transferred from the pixels (or the gates) toward the inductive element and back, all the pixels (or the gates) of one row are snapped to the selecting voltage level during the select time interval. Afterwards all the pixels (or the gates) are discharged again to the deselecting voltage level by means of the same inductive storage element connected to the same voltage node. When the capacitive energy is again transferred from the pixels (or the gates) of one such row toward the inductive element and back, all the pixels (or the gates) of this one row are snapped to the deselecting voltage level during a frame time period. After one row time, the next row of pixels is treated similarly. This cycle repeats each frame time.
Rows/Passive Matrix and Two-Terminal Active Matrix. In another embodiment for this example, the driving scheme allows a voltage pulse to be sent to each row of pixels in turn. Each row is first charged from the deselecting voltage level toward about 2 times the selecting voltage level and immediately back to the deselecting voltage level via an inductive storage element. Again, the inductive storage element is biased at a voltage level between the select and deselect voltage levels. Energy in a capacitive form on the connected row of pixels is transferred to the inductive energy storage element and back towards the capacitive row of pixels. The energy exchange from the capacitive form towards the inductive form and back is repeated an even number of times such that at the end of the select time interval the deselect voltage level is again acquired on the selected row of pixels. A snap circuit can be employed to snap the voltage to the required deselect voltage level after the voltage pulse is fed to one row of pixels. After one row time the next row of pixels is treated similarly. This cycle repeats each frame time.
Rows/Passive Matrix and Two-Terminal Active Matrix. In another embodiment for this example, the inter-row transfer driving scheme allows the deselection and selection of two consecutive rows in a coupled way in turn. A first row is first discharged from the selecting voltage level (xc2x1Vs) toward the deselecting voltage level via an inductive storage element. In this case, the inductive storage element is biased at the deselecting voltage. Energy stored in a capacitive form on the connected row of pixels is transferred to the inductive energy storage element. At that moment when the first row reaches the deselecting voltage level, the next row of pixels is connected to the same side of the same inductive element while the first row of pixels is disconnected. This allows the inductive energy stored in the inductor to be transformed to capacitive energy of the second row of pixels. When the second row of pixels is charged up to the selecting voltage level (}Vs) but reverse polarity with respect to the first row of pixels, the second row of pixels is also disconnected. A snap circuit can be employed to snap the voltage of both rows to the required deselect voltage. After one row time the next couple of rows of pixels is treated similarly. This cycle repeats each frame time. This embodiment implements the row inversion method in a natural way.
The preferred embodiment of the present invention also allows the voltage level of the common node of a three terminal active matrix liquid crystal display to change by connecting it to an inductive element biased at voltage level between the required voltage levels.
Various embodiments of the present invention also include oscillation-sensing circuitry (OSC). An oscillation sensing circuit is added to the different driver schemes to sense the state of the oscillation and to interrupt the oscillation at the appropriate time.